MT Propeller in Germany make propellers for Focke Wolfe 190's, and they have the technology to attach strain gauges to the propeller faces to measure strain in flight, I've seen there set up of this during a factory tour a few years ago. MT know what their time is worth, and it's worth a lot, so don't approach them with a small budget, but they are fully capable.
A strain gauge can also be attached to a spar, but knowing where, and how to resolve the observed data is more complex, and, as you say, requires a co operative warbird owner. Warbirds are high cost, as MT knows their market.